Temperature compensation of tuning forks



Jan. 10, 1967 0.0. ALDERMAN 3,296,918

TEMPERATURE COMPENSATION OF TUNING FORKS Filed June 4, 1964 PPM -'10 60-50 '40 '30 IO 0 1O '20 a0 "IO 1'20 I30 TEMP C.. INVENTOR ORBA D. ALDERMAN $16.2 I BY k 6,0

ATTORNEYS United States Patent 3,296,918 TEMPERATURE COMPENSATIUN 0F TUNING FQRKS Oahu 0. Alderman, Alexandria, Va., assignor to Melpar, Inc, Falls Church, Va., a corporation of Delaware Filed June 4, 1964, Ser. No. 372,461 Claims. (Cl. 84-457) The present invention relates generally to processing tuning forks and more particularly to adjusting the frequency versus temperature characteristic of a tuning fork by adjusting the flux level of magnets associated with the fork drive and pickup coils.

The resonant frequency of tuning forks is known to vary as a function of temperature. This function is generally considered to be a parabola having its axis parallel to the frequency axis for a plot of temperature vs. frequency response. It is generally desired to have the parabola apex positioned at a coordinate on the temperature axis that is commensurate wtih ambient operating conditions for the fork assembly. The frequency coordinate of the apex is fixed by the designed fork frequency and the desired frequency vs. temperature tolerance. The forks, when originally prepared, virtually never have the parabola apex and axis located at the desired parameters. Hence, the forks must be treated in some manner to translate the parabola to a suitable location.

The only prior art method that I know of for shifting the parabola axis to an ambient temperature is heat treating the forks after they have been formed. Heat treating results in translating the axis to a lower temperature and raises the fork resonant frequency. While heat treating translates the axis to the desired temperature for many units, the results attained thereby are not completely predictable in that the axes for many forks are not shifted sufficiently. Insufiicient axis shifting can only be detected after the fork unit has been completely assembled and tested.

When the axis is not shifted enough, the unit must be completely disassembled and the fork subjected to a further heat treating operation. The second heat treating operation may (1) properly locate the axis, (2) not shift it sufficiently or (3) shift it too far. In the second case, the heat treating process must be repeated until the correct temperature coordinate is reached. If the axis is shifted to an excessively low temperature, there is no prior art method I know of for thereafter raising the temperature coordinate of the axis. In the past, units in which the axis was shifted too far were usually discarded. Another disadvantage with the prior art method is that excessive heat treatment can distort the fork tines sufficiently to prevent them from properly oscillating.

I have found that the temperature vs. frequency curve can be translated by varying the magnetic flux density of the magnets associated with the drive and pickup coils of an electromagnetically activated tuning fork. Increasing the magnet flux level results in translating the parabola apex to a lower temperature and resonant frequency while decreases in the magnet flux cause the opposite results. Thus, the present invention enables the temperature axis to be precisely set after the unit has been assembled because the magnet flux fields can be adjusted wtih the unit intact.

In practicing the invention, it has been found most feasible to first heat treat the forks to bring their tempera ture vs. frequency curves into the proper range. The axis of the parabolic curve is then translated to the desired temperature coordinate by adjusting the magnet flux levels. The heat treating operation is preformed because magnets with excessive fiux levels over drive the fork, causing its resonant frequency to change. On the other hand, if the fork is heat treated excessively and the mag- Patented Jan. 10, 1957 net flux levels must be appreciably lowered to arrive at the proper temperature coordinate, the voltage derived from the fork output is not sufficiently large. Thus, the most practical method of adjusting the curve is to heat treat to a point where the temperature axis is higher than the desired value and then adjusting the magnets to have the proper flux level.

Attempts have been made to determine mathematically the relationship between the amount of heat treatment, the magnet flux levels and the desired temperature and resonant frequency of the parabolic apex. Because of the many parameters involved, the attempts have not been successful and empirical data has been collected to determine the degree of heat treating and flux level. In general, it can be stated that the extent of heat treatment and the magnet flux level are directly related to fork resonant frequency.

It is accordingly an object of the present invention to provide a new and improved method for shifting the frequency vs. temperature characteristic of magnetic tuning forks.

Another object of the invention is to provide a method for shifting thefrequency vs. temperature characteristic of magnetic tuning forks without relying exclusively upon heat treating methods.

An additional object of the invention is to provide a method whereby the frequency vs. temperature curve of a tuning fork is shifted while the unit is completely assembled.

A further objectof the present invention is to provide a method that enables tuning fork frequency vs. temperature curves to be shifted to higher temperatures.

Yet an additional object is to provide a method for manufacturing magnetic tuning forks having magnets with flux levels that cause the tines to be neither under nor over driven.

Still a further object of the invention is to provide a method for shifting the temperature vs. frequency curve of a magnetic tuning fork by varying the flux level of the magnets.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction'with the accompanying drawings, wherein:

FIGURE 1 is a view mostly in front elevation and partly in section of a tuning fork with which the present invention can be practiced; and

FIGURE 2 illustrates plots offrequency deviation from desired resonance vs. temperature for the tuning fork of FIGURE 1, wherein frequency deviation is plotted in parts per million (p.p.m.) and temperature in degrees centigrade.

Reference is now made to FIGURE 1 wherein there is illustrated tuning fork assembly 11 comprising vibratile sheet metal tines 12 and 13 of highly permeable magnetic material that are fixedly mounted on strip 14 by spot weld 15. Screws 18 and 19 secure strip 14 to pillars 16 and 17 that are secured to resilient, rubber mountings 21 and 22. Carried at the upper edge of pillars 16 and 17 are fingers 23 and 24 to which bar magnets 25 and 26 are respectively secured. Magnets 25 and 26 are polarized longitudinally, i.e. at right angles to the longitudinal axes of tines 12 and 13. Surrounding magnets 25 and 26 are cylindrical coils 27 and 28, respectively, which coils have their longitudinal axes coincident with the magnet longitudinal axes. Coil 27 is connected to fork input terminals 31 and 32 by leads 33 while coil 28 is connected to fork output terminals 34 and 35 by leads 36. This general tuning fork configuration is disclosed in Patent 3,106,124 to Asten.

Tuning fork assembly 11, including tines 12 and 13, as-

well as mounting strip 14, has a generally parabolic frequency vs. temperature characteristic 41, FIGURE 2, when originally fabricated. It is noted that the apex of curve 41 has a temperature coordinate of approximately 92 C. and a deviation from desired resonant frequency of about 320 parts per million, on the low side. It is desired to translate parabola 41 to the position of parabola 42, having a temperature coordinate of +25 C., room temperature, and a frequency deviation coordinate of minus 250 parts per million. This position of the parabola enables the unit to conform with generally accepted temperature vs. frequency specifications through the 55 C. to +85 C. temperature range.

Curve 41 is initially translated upwardly and to the left to the position of parabola 43 by heat treating fork assembly 11 for a predetermined time at a specified temperature in an inert argon atmosphere. This atmosphere is utilized to prevent fork oxidation, that would adversely affect the magnetic properties of tines 12 and 13.

After assembly 11 is heat treated, it is secured in place i by inserting screws 18 and 19 through strip 14 into pillars 16 and 17. The fiux level of magnets 25 and 26 is then increased to translate curve 43 farther to the left and to a lower position. If the parabola goes too far to the left, as depicted by curve 44, it can be retranslated upwardly and to the right by decreasing the flux level of magnets 25 and 26. After the desired temperature coordinate is reached, i.e. the parabola axis being coincident with +25 C., the curve can be translated upwardly or downwardly by abrading the tines or adding solder to them, as necessary. Changing the mass of the tines in this manner has virtually no effect on the position of the parabolic axis.

It is to be understood that curves 41-44 are not intended to represent curves of an actual device but are merely for purposes of description and example.

The flux of magnet 25 is controlled while it is in situ by first charging it to a very high level. This is accomplished by supplying a high voltage D.C. pulse to coil 27 via terminals 31, 32 and leads 33. Such a pulse is derived by discharging a bank of capacitors, charged to approximately 500 volts, through coil 27. The flux level of magnet 25 is then decreased by connecting a variable amplitude, 60 cycle power source to coil 27 via terminals 31, 32. The 60 cycle current amplitude is increased until the flux level of magnet 25 has decreased to the desired value, as easily determined by inserting a gauss meter probe between tine 12 and coil 27.

An identical procedure is utilized to set magnet 26 to the same flux level as exists in magnet 25. While, ideally, magnets 25 and 26 are charged to the same flux level, it is to be understood that only one of the magnets need be charged. If magnets 25 and 26 have different flux levels, the system oscillates in an unbalanced manner that can be corrected -by changing the mass of one tine.

For a typical example, consider a tuning fork with a desired resonant frequency of 3360 cycles per second in which tines 12, 13 as well as strip 14 are fabricated of NI-SPAN-C strip metal. Tuning fork assembly 11 is first heat treated for two hours at approximately 800 F. in an argon atmosphere. The flux levels of magnets 25 and 26 are then adjusted to approximately 1600 gauss each. It is found that these steps locate the parabola to a position substantially the same as curve 42. While no mathematical function has been found relating the various parameters, in general it can be stated that higher fork resonant frequencies require more heat treatment, and the higher magnet gauss levels.

While I have described and illustrated one specific embodiment of my invention, it will be clear that variations of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

I claim:

1. A method for shifting the temperature vs. frequency curve of a magnetic tuning fork so that a predetermined point on said curve has a specified temperature value comprising heat treating the fork tine assembly to translate said curve to a lower temperature value, then placing said fork tine assembly in the driver assembly, and then adjusting the flux level of the magnets associated with the fork input and output coils until said point has said temperature value by first driving said magnets to a flux level of sufficiently high value that said predetermined point has a temperature coordinate less than said specified temperature value and thereafter decreasing said flux level until said point attains a temperature coordinate of substantially said specified temperature value.

2. The method of claim 1 wherein said magnets are driven to said high value flux level by applying high voltage D.C. pulses to said coils with said magnets in situ, and the flux levels of said magnets are decreased by supplying an increasing AC. voltage to said coils with said magnets in situ.

3. A method for shifting the temperature versus frequency characteristic of a tuning fork assembly having associated therewith fixedly positioned drive and pickup circuits including respective magnets and windings, said method comprising adjusting the magnetic flux intensity of at least one of said magnets by pulsing said at least one magnet with a substantial D.-C. magnetomotive force, and ceasing said pulsing when said characteristic has undergone the desired shift.

4. The method according to claim 3 followed by the step of reducing the flux intensity of said at least one magnet by applying thereto an alternating magnetomotive force of continuously variable amplitude until said characteristic assumes the values of temperature versus frequency desired for operation of said tuning fork.

5. The method according to claim 4 wherein the tuning fork assembly is heat treated prior to said adjustment of magnetic fiux intensity to translate the frequency points. of said characteristic to lower temperature values.

References Cited by the Examiner UNITED STATES PATENTS 2,628,343 2/1953 Murray 84457 X 2,732,748 1/1956 Grib 84-457 2,971,104 2/1961 Holt 84457 X 3,091,151 5/1963 Cunningham 84-457 RICHARD B. WILKINSON, Primary Examiner. C. M. OVERBEY, Assistant Examiner, 

1. A METHOD FOR SHIFTING THE TEMPERATURE VS. FREQUENCY CURVE OF A MAGNETIC TUNING FORK SO THAT A PREDETERMINED POINT ON SAID CURVE HAS A SPECIFIED TEMPERATURE VALUE COMPRISING HEAT TREATING THE FORK TINE ASSEMBLY TO TRANSLATE SAID CURVE TO A LOWER TEMPERATURE VALUE, THEN PLACING SAID FORK TINE ASSEMBLY IN THE DRIVER ASSEMBLY, AND THEN ADJUSTING THE FLUX LEVEL OF THE MAGNETS ASSOCIATED WITH THE FORK INPUT AND OUTPUT COILS UNTIL SAID POINT HAS SAID TEMPERATURE VALUE BY FIRST DRIVING SAID MAGNETS TO A FLUX LEVEL OF SUFFICIENTLY HIGH VALUE THAT SAID PREDETERMINED POINT HAS A TEMPERATURE COORDINATE LESS THAN SAID SPECIFIED TEMPERATURE VALUE AND THEREAFTER DECREASING SAID FLUX LEVEL UNTIL SAID POINT ATTAINS A TEMPERATURE COORDINATE OF SUBSTANTIALLY SAID SPECIFIED TEMPERATURE VALUE. 